Piston position monitor

ABSTRACT

A device is disclosed which maintains the preset position of a reciprocating fluid servomotor. The device includes a spool valve with the spool connected to the servomotor piston rod. Any drifting of the servomotor piston causes the spool to open a port directing pressure fluid to the servomotor to return the piston to its preselected position.

United States Patent Horn 1 [451 Oct. 3, 1972 [54] PISTON POSITIONMONITOR [72] Inventor: John M. Horn, York, Pa.

[73] Assignee: Allis-Chalmers Manufacturing Company, Milwaukee, Wis.

[22] Filed: Oct. 27, 1970 [21] Appl. No.2 84,298

[52] US. Cl ..91/390, 91/466 [51] Int. Cl ..FlSb 13/16 [58] Field ofSearch ..91/390, 368, 387, 466

[56] References Cited UNITED STATES PATENTS 2,966,891 l/l96l Williams..91/387 2,877,743 3/1959 Liaaen ..91/390 3,331,289 7/1967 Horst..91/390 3,568,570 3/1969 Winders ..9 1 I390 Primary ExaminerPaul E.Maslousky Attorney-John P. Hines, Robert B. Charles L. Schwab Benson and7] ABSTRACT A device is disclosed which maintains the preset p0sition ofa reciprocating fluid servomotor. The device includes a spool valve withthe spool connected to the servomotor piston rod. Any drifting of theservomotor piston causes the spool-to open a port directing pressurefluid to the servomotor to return the piston to its preselectedposition.

2 Claims, 1 Drawing Figure PISTON POSITION MONITOR This inventionpertains in general to pressure fluid operated servomotors and moreparticularly to a device for maintaining the preselected position of theservomotor piston.

In all hydraulic or pneumatic servomotors where precise position isrequired, a complete feedback system is used. The feedback system sendsa signal from the servomotors to the main control valve which signal istranslated into movement of the servomotor piston back to itspreselected position. Where the distance between the servomotor and thecontrol is great, the feedback system becomes somewhat complicated. Inmany instances, it would be sufficient or satisfactory to position thedevice if it would remain in the selected position without drifting.However, due to loads that are overcome by the servomotor and inherentleakage in commercial quality systems, there is tendency to drift.

It is therefore the invention and general object of this invention toprovide a monitoring device which will maintain a preselected positionof the servomotor piston without requiring a feedback system to the maincontrol valve.

An additional object of the subject invention is to provide a monitoringdevice of the hereinbefore described type which is located closelyadjacent to the piston which is to be monitored.

A more specific object of the subject invention is to provide amonitoring device of the hereinbefore described type wherein the pistonrod of the servomotor to be monitored is connected directly to andactivates the monitoring device.

An additional object of the subject invention is to provide a pistonmonitoring device of the hereinbefore described type wherein the pistonrod of the servomotor overrides the monitor when it is desired toreposition the servomotor piston, and the monitor is auto- I maticallyreset at the newly selected position of the plston.

These and other objects of the subject invention will become more fullyapparent as the following description is read in light of the attacheddrawing which shows a cross sectional view of a piston position monitorconstructed in accordance with the invention.

Referring to the drawing, the invention is shown for purposes ofillustration in connection with a hydraulic servomotor generallydesignated 6. This servomotor is of the double-acting type having acylinder 7 and a piston 8 reciprocally contained within the cylinder.Although the invention is described in connection with a linearreciprocating piston it has equal application to a rotary reciprocatingpiston. A piston rod 9 attached to the piston extends without thecylinder 7. In the conventional manner in connection with adouble-acting hydraulic servomotor, inlet and discharge ports are 11 and12 are provided on opposite sides of the piston 8. These ports areconnected to a conventional control valve (not shown) which is activatedto selectively position the piston rod 9. The opposite end of the pistonrod is connected to the work which is desired to be moved.

The monitoring device of this invention is positioned closely adjacentto the servomotor 6 so that it can be acted upon by the piston rod 9. Tothis end the monitoring device generally designated 13 may be attacheddirectly to the servomotor cylinder 7. The monitoring device is composedof a valve housing member 14 and a reciprocating valve control member orspool 16 contained in a bore provided within the valve housing. Thespool 16 has a bore therethrough through which the piston rod 9 extends.Means are provided to connect the spool 16 to the rod 9 so that thespool will move with the piston rod but will permit the rod to moverelative to the spool when the spool is held from further movement. Suchconnecting means are herein shown for purposes of illustration as afabric washer or packing 17 provided in a groove in the spool 16. Aconventional packing gland nut 18 is threaded onto the end of the spool16 and can be adjusted to increase or decrease the frictional engagementbetween the spool and the piston rod.

Stop means are provided to limit the extent of movement of the spool 16.To this end and as shown herein for purposes of illustration, a flange19 is provided on the spool 16. A hole is provided through the flange 19through which extends a threaded stud 21. This stud is contained withina threaded bore provided in the valve housing 14. Lock nuts 22 areprovided on the stud so as to selectively adjust the extent of movementof the spool.

The valve housing 14 is provided with a pair of pressure outlet ports 23and 24 which are connected in fluid communication to the servomotorcylinder 7 through passageways 26 and 27 on opposite sides of the piston8. It should be noted that the passageways 26 and 27 connecting thevalve outlets to the servomotor cylinder are much smaller than the maincontrol valve port 11 and 12. This is to insure that the main controlvalve can easily overcome any force exerted by the monitoring valve 13.

The valve housing 14 is also provided with a pair of drain ports 28 and29 which are connected by conduit to a system sump (not shown). Apressure fluid inlet port 31 is also provided in the valve housing 14which is connected by means of a conduit to a source of pressurizedfluid which may be the main system source of pressurized fluid or anauxiliary source.

The spool 16 is provided with an annular pressure chamber 32 and a pairof spaced annular drain chambers 33 and 34 which are isolated from thepressure chamber by means of the annular lands 36 and 37.

OPERATION Assuming that the piston 8 has been moved to the desiredposition as shown in the drawing by the main control valve, theoperation of the monitoring valve is as follows:

If the load on the piston 8 is to the left as shown in the drawing, anyleakage in the system will cause the piston to drift to the left.Because of the frictional connection between the spool 16 and the pistonrod 9, the spool will drift to the left with the piston. This will causethe pressure chamber 32 to uncover the discharge port 23 directingpressure fluid to the lefthand side of the cylinder 7 through thepassageway 26. Pressurized fluid will continue to enter the cylinderuntil the piston 8 moves back to its original position. Since the spool16 moves with the piston rod, the discharge port 23 will be covered bythe land 36 when the piston has been returned to its original piston.

If it is desired to reposition the piston rod 9 to the left, the maincontrol valve will be activated delivering pressurized fluid to theright-hand side of the piston 8. This will cause the piston rod and thespool 16 to move to the left until the flange 19 contacts the lock nuts22. At this point the force on the piston will overcome the holdingforce of the frictional engaging means permitting the piston to be movedfurther to the left to its new desired location. Since the spool is nowengaging the lock nuts 22, fluid from the monitoring device will enterthe left side of the cylinder 7 only very slightly causing the piston tomove to the right until the spool recenters itself in the position shownin the drawing. From that point, the monitoring device will operate inthe manner above described maintaining the piston in the newly locatedposition.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A position monitoring device for a pressure fluid operated servomotorhaving a cylinder, a piston slidable therein and a piston rod connectedto the piston and extending to the exterior of the cylinder, said devicecomprising: a housing connected to said cylinder defining a chamberthrough which said piston rod extends; a pressurized fluid inlet port insaid housing connected in communication to a source ofpressurized fluid,a pressure fluid delivery port in said housing connected in fluidcommunication to a said cylinder; a control spool located within saidhousing in a manner to permit movement thereof relative to said housing;stop means associated with said housing and said control spool limitingmovement therebetween to a predetermined amount; attaching meansconnecting said control spool to said piston rod for limited movementtherewith within said chamber, said attaching means adjusted to permitmovement of said piston rod relative to said control spool only aftersaid stop means is engaged; and a passageway in said control spoolselectively allignable with said housing ports upon movement of saidpiston rod from its set position to connect said pressure fluid to saidcylinder to return said piston to its set position.

2. The position monitoring device set forth in claim 1 wherein saidspool has a bore therethrough through which said piston rod extends andfurther comprising: an annular frictional packing surrounding saidpiston rod and engaging said spool; and a pressure nut threaded on saidspool surrounding said piston rod and engaging said packing toselectively adjust the holding force between said piston rod and saidspool.

1. A position monitoring device for a pressure fluid operated servomotorhaving a cylinder, a piston slidable therein and a piston rod connectedto the piston and extending to the exterior of the cylinder, said devicecomprising: a housing connected to said cylinder defining a chamberthrough which said piston rod extends; a pressurized fluid inlet port insaid housing connected in communication to a source of pressurizedfluid, a pressure fluid delivery port in said housing connected in fluidcommunication to a said cylinder; a control spool located within saidhousing in a manner to permit movement thereof relative to said housing;stop means associated with said housing and said control spool limitingmovement therebetween to a predetermined amount; attaching meansconnecting said control spool to said piston rod for limited movementtherewith within said chamber, said attaching means adjusted to permitmovement of said piston rod relative to said control spool only aftersaid stop means is engaged; and a passageway in said control spoolselectively allignable with said housing ports upon movement of saidpiston rod from its set position to connect said pressure fluid to saidcylinder to return said piston to its set position.
 2. The positionmonitoring device set forth in claim 1 wherein said spool has a boretherethrough through which said piston rod extends and furthercomprising: an annular frictional packing surrounding said piston rodand engaging said spool; and a pressure nut threaded on said spoolsurrounding said piston rod and engaging said packing to selectivelyadjust the holding force between said piston rod and said spool.